1. Field of the Invention
The present invention relates to down-hole tools and particularly to stabilisers for drill strings, especially near-bit stabilisers.
2. Description of the Related Art
Directional drilling is either sophisticated, expensive and unreliable or simple, reliable but rather limited. For the most part, the latter type meets all requirements. This type relies entirely on gravity and can only adjust the inclination of a hole, rather than its horizontal direction.
An adjustable stabiliser has a base diameter larger than the drill string, but not as large as the hole bore being drilled. It prevents the drill string from contacting the sides of the bore. When actuated however, its diameter increases and so the drill string is constrained to run concentric with the hole being drilled. Thus an adjustable stabiliser near the drill bit steers the drill bit depending on its actuation.
Down-hole motors are frequently used in drilling. The string itself is not rotated. Instead, the motor near the end of the string rotates just the bit at the end. The motor is hydraulically driven by drilling mud pumped from the surface. The down-hole motor should be as close to the drill bit as possible, but a stabiliser can be interposed between them in order to provide steerage.
Thus a short stabiliser is called for. Down-hole stabilisers have been actuated in a number of different ways.
In EP-A-0251543, a fairly short stabiliser is disclosed, but it involves using mechanical compressive forces on the drill string to set and unset it.
In U.S. Pat. No. 4,951,760, a stabiliser is hydraulically operated, employing fluid pressure of pressurised drilling mud to actuate the stabiliser by a piston mandrel moving axially in a bore of the body of the stabiliser and having ramps or cams which move a stabiliser bar radially outward. A long, and strong, spring returns the stabiliser to a deactivated position when the fluid pressure is released.
In the same patent a throttle member increases the pressure drop across the tool, serving both to accelerate movement of the mandrel for actuation of the tool and to signal to the surface the state of actuation of the tool.